lib-index: fsck: Fix small log offsets to file's correct header size

Use the file's actual current header size, not
MAIL_TRANSACTION_LOG_HEADER_MIN_SIZE, which is nowadays smaller. This
resulted in unnecessary errors like:

Corrupted transaction log file ...: Invalid min_file_offset: ..., min_file_offset (24) < hdr_size (40)

src/lib-index/mail-index-fsck.c

@@ -29,6 +29,7 @@ static void
 mail_index_fsck_log_pos(struct mail_index *index, struct mail_index_map *map,
 			struct mail_index_header *hdr)
 {
+	unsigned int hdr_size = index->log->head->hdr.hdr_size;
 	uint32_t file_seq;
 	uoff_t file_offset;
 
@@ -42,14 +43,14 @@ mail_index_fsck_log_pos(struct mail_index *index, struct mail_index_map *map,
 		   offsets are valid. */
 		if (hdr->log_file_head_offset > file_offset)
 			hdr->log_file_head_offset = file_offset;
-		else if (hdr->log_file_head_offset < MAIL_TRANSACTION_LOG_HEADER_MIN_SIZE)
-			hdr->log_file_head_offset = MAIL_TRANSACTION_LOG_HEADER_MIN_SIZE;
+		else if (hdr->log_file_head_offset < hdr_size)
+			hdr->log_file_head_offset = hdr_size;
 
 		if (hdr->log_file_tail_offset > hdr->log_file_head_offset)
 			hdr->log_file_tail_offset = hdr->log_file_head_offset;
 		else if (hdr->log_file_tail_offset != 0 &&
-			 hdr->log_file_tail_offset < MAIL_TRANSACTION_LOG_HEADER_MIN_SIZE)
-			hdr->log_file_tail_offset = MAIL_TRANSACTION_LOG_HEADER_MIN_SIZE;
+			 hdr->log_file_tail_offset < hdr_size)
+			hdr->log_file_tail_offset = hdr_size;
 	} else {
 		/* index's log_file_seq is newer than exists. move it to
 		   end of the current log head. */